The bonding or welding of lead wires between an integrated circuit chip or die and the lead frame on which the die is mounted for coupling to external circuitry is presently accomplished by manual, semi-automatic and automatic lead wire bonding machines. A fine lead wire such as 1 mil diameter gold bonding wire is held in a capillary bonding tool so that the lead wire projects beyond the end of the tool. The bonding tool is mounted in a tool holder arm which is in turn a component of the movable bonding head of the bonding machine. The bonding tool is appropriately mounted over a strip of lead frames retained within the guides of the bonding machine work holder. At a finer level, the bonding tool is positioned over the metalized die pad of an integrated circuit chip mounted on one of the lead frames, and then over a lead frame finger.
Examples of such lead wire bonding machines include the Model 478, High Speed Tailless Thermocompression Ball Bonder, a manual or semi-automatic model manufactured by Kulicke & Soffa Industries, Inc. (K&S), Horsham, Penn. 19044, described in U.S. Pat. No. 3,643,321; the K&S Model 1418/1419, Automatic High Speed Wire Bonder With Digital Bonding Head; and K&S Model 1482, Automatic Wire Bonder. Other examples of state of the art lead wire bonding machines are those of The Welding Institute, Abington, England, described in U.S. Pat. Nos. 4,323,759 and 4,098,447.
Bonding of lead wire between the die pad of an integrated circuit chip and a lead frame finger is generally accomplished by a ball bond wedge bond cycle. A spherical ball is formed at the end of the lead wire extending below the capillary bonding tool by, for example, arc discharge between the bonding wire and a shield or shroud electrode. After solidification, the metal ball at the end of the lead wire is brought into intimate contact with the metalized die pad and a bond is formed typically by application of ultrasonic bonding energy to the bonding tool. Thermocompression is also utilized during bonding by maintaining the work holder at an elevated temperature and by applying a specified bonding force to the bonding tool.
The capillary bonding tool is then raised to a level above the ball bond and die with the lead wire feeding through the capillary passageway in the bonding tool. The bonding tool and lead frame are then moved relative to each other for bonding of a segment of the lead wire spaced from the ball bond at another location on a lead frame finger. At this new location the lead wire is brought into intimate contact with the surface of a lead frame finger to form a so-called "wedge bond" or "weld". The wedge bond is formed by the side tip of the bonding tool bearing down on the lead wire against the surface of the lead frame finger. Again, the bond is typically formed by application of ultrasonic bonding energy to the bonding tool. Thermocompression also operates to form the wedge bond. Typically the second bond force applied to the bonding tool for the wedge bond or weld is different from and greater than the first bond force applied to the bonding tool for ball bonding.
In the case of both the ball bond and wedge bond the application of ultrasonic bonding energy to the bonding tool is terminated before further movement of the tool. The joining of the bonded or welded surfaces is therefore completed before the bonding tool is raised. The lead wire is then severed above the wedge bond by clamping the lead wire at a clamp on the bonding head and raising the bonding head, bonding tool and lead wire above the wedge bond so that the lead wire parts at the weakened neck adjacent to the wedge bond. Further background on ball bonding and wedge bonding can be found in the cross referenced U.S. Pat. No. 4,555,052, referred to above, and U.S. Pat. No. 4,390,771.
In performing a ball bond wedge bond cycle, the ball bonding machine typically applies a first bond force to the bonding tool during ball bonding in the order of, for example, approximately 30 to 50 grams according to the size and metal composition of lead wire. During wedge bonding, a second bond force is applied to the bonding tool greater than the first bond force, for example, in the order of approximately 80 to 100 grams or greater according to the diameter of the wire and the composition of the lead wire metal, for pressing, weakening, and partially cutting the lead wire at the end of the wedge bond. The lead wire may then be cleanly severed or parted at the weakened edge by clamping the lead wire in the bonding head and raising the bonding head and bonding tool.
The first and second bonding forces at the bonding tool are typically applied by either the weight of components resting on the bonding tool or tool holder, application of specified spring tension to the bonding tool, tool holder or tool lifter, or the driving force of a drive motor applied to the bonding head or bonding tool supporting components. A disadvantage of presently available ball bonding machines is that the application of ultrasonic bonding energy typically occurs after application of a specified bonding force. Thus, ultrasonic bonding energy is applied to the tool after the lead wire is brought to bear against the die pad or lead frame finger substrate with a force in the order of, for example, 25 grams.
The manner of sensing the threshold level of applied bonding force in the order of, for example, 25 grams is illustrated in the diagrammatic representations of a prior art K&S bonding machine shown in FIGS. 1 and 2. The schematically represented bonding tool 10 is supported in a tool holder or tool holding arm 12 which in turn constitutes part of the movable bonding head of the bonding machine. A tool holder lifter or tool holder lifting arm 14, also part of the bonding head, raises and lowers the tool holder 12 but with some play or relative movement between the tool holder lifter 14 and tool holder 12. During downward movement the tool holder lifter 14 and tool holder 12 are coupled by a spring coupling 15 having a spring constant K selected to impart a desired bonding force to the bonding tool 10. The tool holder lifter 14 is raised and lowered by a servo motor driven crank 16 through an eccentrically mounted connecting rod 17 and related linkages and couplings. The servo motor operating crank 16 and connecting rod 17 are driven by a servo motor and control loop hereafter described with reference to FIG. 5B. During downward motion of the tool holder lifter 14 and tool holder 12 driven by connecting rod 17 for ball bonding or wedge bonding, the bonding tool is applied against the substrate 18, either the die pad of an integrated circuit chip or a lead frame finger with the desired bonding force proportional to the spring constant K and the depth of compression of the spring in turn determined by the stroke of rod 17.
The bonding tool holder 12 rests against the tool holder lifter 14 at electrical contacts 20 which are connected in an electrical circuit not shown which provides an electrical sensor. The electrical contacts 20 are closed during vertically upward motion of the bonding tool, that is during most of the upward stroke of connecting rod 17. Furthermore, the electrical contacts are closed during the downward stroke until the applied compressional force on the bond force spring 15 initiates compression of the spring separating the contacts 20 so that the contacts are open. The elements are typically constructed and arranged so that the contacts 20 open when the applied compressional force or bonding force reaches a level of, for example, in the order of 25 grams.
Upon opening of the contacts the electrical circuit and sensor not shown turns on the ultrasonic generator of the bonding machine and initiates application of ultrasonic bonding energy from the ultrasonic generator to the tool holder and bonding tool 10. As a result the control of ultrasonic power is dependent upon the application of mechanical force and the opening of mechanical contacts.
A disadvantage of the prior art method of mechanical sensing for initiating ultrasonic bonding energy and ultrasonic vibration of the lead wire against the pertinent substrate is that an incomplete or reduced welding or bonding area results. The limited and inferior welding or bonding surface area which results from this method of initiating and controlling ultrasonic bonding energy is illustrated in the diagrammatic plan view of FIG. 3 showing a ball bond site 22 after removal of the compressed bonding ball from the die pad 24 of an integrated circuit chip. The bonding surface area 25 shown in irregular lines is limited to a peripheral band around the circumference of the circular ball bond site. This is because ultrasonic bonding energy arrives at the tool initiating ultrasonic vibration of the compressing surfaces only after contact has been made by the center of the ball at the substrate and after substantial compression of the ball. By that time, the major force bearing region of contact is at the peripheral band enlarging outward as the spherical bonding ball is compressed and flattened against the die pad substrate. Frequently only a minor portion of the available bonding surface actually participates in a co-mingling weld between the two metals of the lead wire and substrate. In effect, the welding surface area or bonding surface area is limited to the area of coincidence of effective thermocompression or bonding force with ultrasonic vibration from application of ultrasonic bonding energy. As a result, a substantial central area 26 in the middle of the ball bonding site remains unwelded.
The same limitation on the welding or bonding surface area occurs at the wedge type bond as illustrated in the diagrammatic perspective view of FIG. 4 where a wedge-type bond is being parted by lifting the lead wire 30 from the lead frame finger 32. The effective welding or bonding surface area 33 is limited to a peripheral band of elongate or oval configuration at the periphery of the elongate wedge bond 28. The central portion 34 of the lead wire 30 at the wedge bond site remains unwelded to the substrate.
Because of the failure of welding at the center of both the ball bond and the wedge bond, weldment strength suffers and the maximum pull strength at both bonds is reduced. Applicant's experimental analysis indicates that this sub-optimal bonding and welding prevails because of the substantial delay from the application of first or second bonding forces with compression of the lead wire at the ball bond and wedge bond to the subsequent application of ultrasonic bonding energy. The mechanical methods of sensing bond force for initiation and control further delays the application and onset of ultrasonic bonding energy.